This invention relates generally to the subject matter of Argentine Pat. No. 226,607 of applicant, filed July 13, 1981 and issued July 30, 1982 for Broca Para Recuperar Muestras Geologicas.
The present invention refers to a drill bit for the recovery of geological samples, such as rocks in generally cylindrical bar form, typically referred to as a core. The geological samples are generated continuously during drilling operations and are efficiently transported to the surface for inspection by geologists at any given time during drilling operations. The continuous coring drill bit of the present invention lends itself efficiently to the use of reverse circulation of drilling fluid for accomplishing continuous drilling and coring operations.
In general terms, there are two systems for recovering geological samples, using a widely varied design for drill bits for drilling rocks. The first system, referred to as direct circulation, consists of injecting a fluid, commonly a liquid mud drilling or weighting through a length of pipe, one end of which is connected to a rotating system capable of rotating it around its long axis. The lower extremity of the rotating drill pipe is connected to the upper portion of a drill bit. The drilling fluid is injected into the bottom of the drilled hole through openings in the lower portion of the bit and returns to the surface through the annulus defined between the rotating drill pipe and the wall surface of the well bore which is being drilled.
In this direct circulation system it is necessary to continue casing the hole being drilled to insure that the particles cut from the formation are carried upwards by the fluid on its return without being plastered into the wall of the borehole. If the particles cut are very small, then the muddy and abrasive fluid acts to further reduce such particles while they are traveling to the surface along with the drilling fluid. The rotating drill pipe can engage the wall surface of the well bore during rotation and, in such event, tends to crush the drilled particles, thus further reducing them in size as they continue upwardly through the annulus. Considering that some boreholes are very deep, the excessive reduction of such rock particles make it difficult to obtain a reliable concept of the relationship of formations and the depth from which the particles originated. This provides significant problems to geologists who must interpolate the particle samples to identify the characteristics of the formation being drilled.
As the well bore nears a high pressure oil or gas production zone, it is very important that geologists be able to accurately identify the character of the formation being drilled. For this reason, well drilling operations in deep wells typically require cores to be provided for inspection by geologists. For coring, the drill pipe is removed from the well and a drilling device, typically referred to as a core barrel, is attached to the drill pipe in place of the drill bit. After drilling to a particular depth, the drill pipe and core barrel are removed from the well bore, the core barrel containing a core, which is an unbroken section of rock formation. The core is processed by geologists who assist in rendering decisions for further well drilling operations. When core barrels are employed for coring operations, the drill string must be removed from the well bore at the time the core barrel is installed and must again be removed from the well bore after the core barrel has reached a depth filling its internal chamber with core material. Obviously, frequent removal of the drill string from the well bore for coring is an expensive and time consuming operation which should be avoided if possible. Through employment of the continuous coring drill bit of this invention, it is not necessary to frequently remove the drill pipe from the well bore and yet, if the core material is proper for purposes of geological inspection, the results of coring will be quite satisfactory.
Another system for recovery of geological samples from wells being drilled is known as reverse circulation. In this case, well drilling mud or washing fluid is forced downwardly from the surface in the annulus between the rotating drill pipe and the wall surface of the well being drilled. When it reaches the bottom of the hole, it enters appropriate openings of the drill bit and then traverses upwardly through the drill pipe to the surface carrying with it drill cuttings that are removed from the formation being drilled by the rotating drill bit. For recovery of geological samples, a continuous coring-type drill bit may be employed which accomplishes formation of a core at the time of drilling and fractures the core into relatively small pieces which are transported to the surface, along with the upwardly moving drilling fluid in the drill pipe. At the surface, separators such as shale shakers are employed to separate the core material from the drilling fluid, thus permitting geologists to frequently recover core samples and analyze them for their termination of the character of the formation being drilled.
From the foregoing, it appears that the system of reverse circulation provides improved possibilities of geological investigation as it provides for the recovery of rock samples which are of large and consolidated form thus providing geologists with the capability of efficiently determining the character of the formation being drilled. Since frequent interchange of drill bits and core barrels is eliminated, the drilling operation can continue at a rapid rate and at low cost without jeopardizing the drilling operation from the standpoint of geological efficiency.
Although the continuous coring aspects of the present invention may be utilized in drilling systems of typical circulation or direct circulation where drilling fluid is forced downwardly through the drill stem to the drill bit and returns via the annulus between the drill stem and borehole wall, better geological results are obtained through use of the continuous coring drill bit of this invention in conjunction with reverse circulation. In this case, the drilling fluid is forced downwardly in the annulus between the drill stem and the wall of the borehole. As the drill bit is rotated against the formation, drilling fluid from the annulus flows past the cutting surface and enters the throat of the drill bit. It then flows upwardly through the internal cavity or passage defined by the drill bit and then flows upwardly through the drill stem to the surface. The core particles which are fractured by the floating core breaker are transported upwardly through the drill stem along with the drilling fluid. The core particles will separate out readily and continuously by the shale shaker of the drilling rig and may be obtained and analyzed by geologists at any given time in the drilling operation. The geologists are enabled thereby to continuously monitor the character of the formation being drilled. This enables the parameters of drilling operations such as mud consistency, drill bit weight, etc., to be modified as is appropriate to the specific conditions existing at the location of the drill bit.
Through employment of reverse circulation, much more efficient control of the well being drilled may be maintained in the event a gas pocket is encountered. If a quantity of gas enters a well being drilled through employment of conventional circulation of drilling fluid, the gas expands as it rises toward the surface and occupies a greater volume of space because the hydrostatic head of the drilling fluid column is reduced. This is typically known as "kicking of the well". If this situation is not carefully controlled at the time of well kicking, a blow out can occur, thus endangering equipment and personnel as well as adversely affecting the character of the producing formation. Upon employment of reverse circulation procedures, the column of expanding gas is entrapped within the drill stem which is capable of resisting high pressure and it is efficiently controllable by surface equipment. In the event a well should start kicking during drilling operations it may be simply and efficiently controlled by reverse circulation procedures where in ordinary circumstances control could be difficult or impossible.